Thin buried oxides by low-dose oxygen implantation into modified silicon

ABSTRACT

A method of fabricating silicon-on-insulators (SOIs) having a thin, but uniform buried oxide region beneath a Si-containing over-layer is provided. The SOI structures are fabricated by first modifying a surface of a Si-containing substrate to contain a large concentration of vacancies or voids. Next, a Si-containing layer is typically, but not always, formed atop the substrate and then oxygen ions are implanted into the structure utilizing a low-oxygen dose. The structure is then annealed to convert the implanted oxygen ions into a thin, but uniform thermal buried oxide region.

FIELD OF THE INVENTION

The present invention relates to a method of fabricating a semiconductorstructure, and more particularly to a method of fabricating asilicon-on-insulator (SOI) having a thin, uniform buried oxide, lessthan 100 nm in thickness, that is formed by using low-dose oxygenimplantation.

BACKGROUND OF THE INVENTION

Silicon-on-insulator (SOI) structures are used in microelectronic deviceapplications where the electrical and electronic interactions betweenthe active device region and the underlying semiconductor structure arestrongly discouraged. In a typical SOI structure, the buried oxide layerseparates the Si over-layer (i.e., the SOI or device layer) from the Sisubstrate.

In complementary metal oxide semiconductor (CMOS) devices built on SOI,for instance, performance characteristics are known to be greatlyimproved. Specifically CMOS devices built on SOI can exhibit lessjunction capacitance and leakage, greater resistance to ionizingradiation, immunity to latch-up, etc. However, forming SOI structures isno simple matter.

Even after decades of research and development only a few methods areproven to be commercially viable. In one, called BESOI(bond-and-etch-back SOI), two Si wafers are oxidized at the surface andthe oxidized surfaces are bonded together and then one of the two bondedwafers is etched to provide a thin SOI device layer. In this prior artmethod and its variations, as the wafer surfaces are oxidized beforebonding, the buried oxide can be made to have any desired thickness.However, impurities at the bonded interface and the difficulty inachieving a thin, uniform Si over-layer through the etch-back processare major drawbacks. The terms “Si over-layer” and “SOI layer” may beinterchangeably used in this application.

In another well-known method, called SIMOX (separation by implantationof oxygen), a selected dose of oxygen ions is directly implanted into aSi wafer, and then the wafer is annealed in an oxygen ambient at a hightemperature so that the implanted oxygen is converted into a continuousburied oxide layer. The thickness of the buried oxide layer in the SIMOXmethod is mostly dependent on the implanted oxygen dose and the thermaloxidation conditions. Moreover, in SIMOX, the Si over-layer is thinnedto a desired thickness during the thermal oxidation, after which thesurface oxide is stripped off.

When the peak concentration of the implanted oxygen is very low (on theorder of about 1E22 atoms/cm³ or less), however, the buried oxidetypically becomes broken and discontinuous, as the growing oxideprecipitates tend to ball up to minimize the surface energy. Such an SOIstructure is shown, for example, in FIG. 1. In FIG. 1, reference numeral100 denotes the Si-containing substrate layer, reference numeral 102denotes the buried oxide and reference numeral 104 denotes theSi-containing over-layer of a prior art SOI structure. As such, it isgenerally very difficult to form a buried oxide layer thinner than 100nm using conventional SIMOX processing.

In MOSFET device applications, the Si over-layer and the buried oxideunderneath need to be made thinner as the device dimensions shrink, inorder to better control short-channel effects. This means that theburied oxide in up coming generations of MOSFET devices needs to be farthinner than what the conventional SIMOX technology is capable of.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a method offabricating SOI structures with a very thin (less than 100 nm), butuniform buried oxide.

Another object of the present invention is to provide a method offabricating SOI structures in which the processing time is reduced, yetthe throughput is increased by reducing the oxygen implantation dose toa level below which is allowed for in a typical SIMOX process.

An even further object of the present invention is to provide a methodof fabricating SOI structures where the defect level in the Siover-layer, i.e., the SOI layer, is reduced by reducing implantationdamages and by reducing the stresses and strains from the expandingvolume of the buried oxide.

These and other objects and advantages are achieved in the presentinvention by utilizing a method wherein a low-dose oxygen implantationstep is performed. By “low-dose”, it is meant an oxygen dose of about1E17 atoms/cm² or less. In prior art SIMOX processes, a low-dose oxygenimplantation would usually result in a layer of broken and discontinuousburied oxides as the oxide tends to ball up to minimize surface energy.In the present invention, the problem is solved by forming a largenumber of vacancies or voids in a Si-containing substrate prior to thelow-dose oxygen implantation step.

The vacancies or voids coalesce during the subsequent high-temperatureoxidation and provide room for the buried oxide to expand laterally,resulting in a thin, uniform buried oxide layer. The term “uniform” isused in the present invention to denote a buried oxide region having acontinuous interface with the Si-containing over-layer as well as theunderlying Si-containing substrate wherein the variation of thicknessacross the entire wafer is less than 30% of the total thickness of theburied oxide layer. With a sufficient density of vacancies or voids, thethickness of the buried oxide layer is mostly dependent on the implanteddose and the internal thermal oxidation conditions. In one embodiment ofthe present invention, the vacancies or voids are formed into aSi-containing substrate by utilizing an electrolytic anodization processwherein a HF-containing solution is used.

In accordance with the method of the present invention, the SOIstructure is fabricated by modifying a surface of a Si-containingsubstrate to contain a large concentration (on the order of about 0.01%or greater) of vacancies or voids. The terms “vacancies” and “voids” areinterchangeably used in the present invention to denote a porous Siregion. Next, a Si-containing layer is typically, but not always, formedatop the substrate and then oxygen ions are implanted into the structureutilizing a low-oxygen dose. The structure is then annealed to convertthe implanted oxygen ions into a thin, but uniform thermal buried oxideregion.

In broad terms, the method of the present invention comprises the stepsof:

providing a structure comprising at least a Si-containing substrate thathas a region of vacancies or voids located therein;

optionally forming a single crystal Si-containing layer atop saidstructure;

implanting oxygen ions into said structure using an oxygen dose of about1E17 atoms/cm² or less; and

annealing the structure containing implanted oxygen ions and vacanciesor voids to form a silicon-on-insulator that includes a Si-containingover-layer and a buried oxide, said buried oxide having a thickness ofabout 100 nm or less.

In some embodiments of the present invention, a bake step conducted in ahydrogen-containing ambient is employed prior to the optional forming asingle crystal Si-containing layer over the porous structure or prior tothe implanting step.

In yet another embodiment of the present invention, a baking stepperformed in a hydrogen-containing atmosphere is employed to the SOIstructure which includes the thin buried oxide layer.

The SOI structure obtained from the present invention contains a verythin, yet uniform and continuous buried oxide region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial representation (through a cross sectional view) ofa prior art SOI structure made from a conventional SIMOX process using alow-dose oxygen implantation step.

FIGS. 2A-2H are pictorial representations (through cross sectionalviews) illustrating the basic processing steps of the present invention.

FIGS. 3A-3C are cross-sectional SEM images through various processingsteps of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention, which provides a simple and low-cost method forforming an SOI substrate having a thin, uniform buried oxide regionunderlying a Si-containing overlayer, will now be described in greaterdetail by referring to the drawings that accompany the presentapplication. In the accompanying drawings, like and/or correspondingelements are referred to by like reference numerals.

Reference is first made to the initial structure shown in FIG. 2A, whichincludes a Si-containing substrate 10 having a region 12 of vacancies orvoids formed therein. The terms “vacancies” and “voids” areinterchangeable used in the present invention to denote a porousSi-containing region. The term “Si-containing” as used herein denotes asemiconductor material that includes at least silicon. Illustrativeexamples of such Si-containing materials include, but are not limitedto: Si, SiGe, SiC, SiGeC, epi-Si/Si, epi-Si/SiC, epi-Si/SiGe, andpreformed silicon-on-insulators (SOIs) or SiGe-on-insulators (SGOIs)which may include any number of buried insulating (i.e., continuous,non-continuous or a combination of continuous and non-continuous)regions formed therein.

The Si-containing substrate is a doped substrate that may contain a p-or n-type dopant, with p-type dopants being highly preferred. Doping isachieved either by growing doped Si ingots from which p- or n-dopedwafers are cut and polished, or by ion implantation. Both of theaforementioned doping processes are well known to those skilled in theart. The concentration of dopants within the initial Si-containingsubstrate 10 may vary depending on the dopant used. For n-type dopants,the concentration of dopant being implanted is typically from about 1E17to about 1E18 atoms/cm³, whereas for p-type dopants, the concentrationof dopant being implanted is typically from about 1E15 to about 2E19atoms/cm³.

Region 12 is formed near a surface region of the Si-containing substrate10 using an electrolytic anodization process that is capable of forminga porous Si-containing region in the Si-containing substrate 10. Theporous Si-containing region, i.e., region 12, includes vacancies orvoids therein. The anodization process is performed by immersing thestructure shown in FIG. 2A into an HF-containing solution while anelectrical bias is applied to the structure with respect to an electrodealso placed in the HF-containing solution. In such a process, the p-typestructure typically serves as the positive electrode of theelectrochemical cell, while another semiconducting material such as Si,or a metal is employed as the negative electrode.

In general, the HF anodization converts doped single crystal Si intoporous Si. The rate of formation and the nature of the porous Siso-formed (porosity and microstructure) is determined by both thematerial properties, i.e., doping type and concentration, as well as thereaction conditions of the anodization process itself (current density,bias, illumination and additives in the HF-containing solution).Specifically, the porous Si forms with greatly increased efficiency inthe higher doped regions.

Generally, the porous Si-containing region 12 formed in the presentinvention has a porosity of about 0.01% or higher. The depth of theporous Si-containing region 12 is typically from about 1000 nm or less,as measured from the upper most surface layer of the Si-containingsubstrate 10.

The term “HF-containing solution” includes concentrated HF (49%),concentrated HF with acetic acid, a mixture of HF and water, a mixtureof HF and a monohydric alcohol such as methanol, ethanol, propanol, etc,or HF mixed with at least one surfactant. The amount of surfactant thatis present in the HF solution is typically from about 1 to about 50%,based on 49% HF.

The anodization process, which converts a near surface portion of theSi-containing substrate 10 into a porous Si-containing region 12 using aconstant current source that operates at a current density of from about0.05 to about 50 milliAmps/cm². A light source may be optionally used toilluminate the sample. More preferably, the anodization process of thepresent invention is employed using a constant current source operatingat a current density of from about 0.1 to about 5 milliAmps/cm².

The anodization process is typically performed at room temperature or ata temperature that is elevated from room temperature may be used.Following the anodization process, the structure is typically rinsedwith deionized water and dried.

In an optional embodiment of the present invention, the structure shownin FIG. 2A may be baked in a hydrogen-containing ambient at atmosphericor a reduced pressure at this point of the present invention. Whenperformed, this optional embodiment desorbs impurity atoms from theporous Si-containing region 12 (i.e., region containing vacancies andvoids), while at the same time closing up any surface pores. The bake ina hydrogen-containing ambient is performed at a temperature from about800° to about 1200° C., with a temperature from about 1000° to about1150° C. being more highly preferred. Examples of hydrogen-containingambients include H₂, NH₄, or mixtures thereof, including mixtures with,or without, an inert gas.

Next, a single crystal Si-containing layer 14 is typically, but notalways, formed atop the Si-containing substrate 10 containing the porousSi-containing region 12 at this point of the present invention. Thesingle crystal Si-containing layer 14 may not be needed when the porousSi-containing region 12 is formed some distance, 50 nm or greater, belowthe surface of the Si-containing substrate 10. The structure includingthe single crystal Si-containing layer 14 is shown, for example, in FIG.2B; reference numeral 13 denotes the interface between the porousSi-containing region 12 and the single crystal Si-containing layer 14.The single crystal Si-containing layer 14 employed in the presentinvention comprises any Si-containing material including, for example,epitaxial Si (epi-Si), amorphous Si (a:Si), SiGe, single orpolycrystalline Si or any combination thereof. Of the various Simaterials listed above, it is preferred that epi-Si or epi-SiGe beemployed as the single crystal Si-containing layer 14.

The single crystal Si-containing layer 14 has a thickness of from about1 to about 1000 nm, with a thickness of from about 1 to about 400 nmbeing more highly preferred. The single crystal Si-containing layer 14is formed using known deposition processes including an epitaxial growthprocess.

The structure including the thus formed porous Si-containing region 12,with the single crystal Si-containing layer 14 is then implanted withoxygen ions. The implant step may be a blanket implant in which oxygenions are implanted across the entire wafer. This embodiment is depictedin FIG. 2C wherein region 16 denotes the oxygen implant region. Theoxygen implant step may vary such that the oxygen peak is located at theSi-containing layer/porous Si interface or within the porous Si region(not shown). FIG. 2D depicts an embodiment in which a patterned oxygenion implant step is performed forming patterned regions of implantedoxygen ions. Reference numeral 16′ denotes the patterned regions ofimplanted oxygen ions.

The oxygen implant step is preformed using a low-dose implantationprocess. By “low-dose” it is meant an implant process in which the doseof oxygen ions being implanted into the Si-containing structures isabout 1E17 atoms/cm² or less at greater than 200° C. More preferably,the oxygen implant step of the present invention is performed using adose of oxygen ions from about 1E16 to about 5E16 atoms/cm². The implantmay be performed in a continuous mode, or the implant may be performedusing a pulse mode.

The low-dose oxygen ion implant step of the present invention isperformed using a conventional implanter in which a beam current densityfrom about 0.05 to about 500 milliAmps/cm², with a beam current densityfrom about 5 to about 50 milliAmps/cm² being more typical, is employed.The low-dose oxygen implant step of the present invention is typicallyperformed at a temperature from about 200° to about 600° C. Moretypically, the temperature in which the implant is performed is fromabout 200° to about 400° C. The implant is performed at an energy fromabout 40 to about 1000 keV, with an energy from about 100 to about 200keV being more typical.

In addition to the foregoing base oxygen implant step, an optionalsecond oxygen implant step may be performed to enhance the uniformity ofthe buried oxide to be subsequently formed. The optional second oxygenimplant step is performed at a dose of about 1E17 atoms/cm² or less.More preferably, the optional second oxygen implant step is performedusing a dose of oxygen ions from about 1E14 to about 1E16 atoms/cm². Theimplant may be performed in a continuous mode, or the implant may beperformed using a pulse mode.

The optional second oxygen ion implant step of the present invention isperformed using a beam current density from about 0.05 to about 5milliAmps/cm². The optional second oxygen implant step of the presentinvention is typically performed at a temperature from about 4K to about200° C. More typically, the temperature in which the optional implant isperformed is from about nominal room temperature to about 100° C. Theoptional implant step is performed at an energy from about 40 to about1000 keV, with an energy from about 100 to about 200 keV being moretypical.

The low-dose oxygen implant step forms an oxygen implant region 16 thathas a depth, as measured from the upper surface of the Si-containingsubstrate 10 of about 1500 nm or less. More preferably, the depth of theoxygen implant region 16 is from about 100 to about 500 nm. The depth ofthe oxygen implant region 16 should preferably be at the center orslightly below interface 13.

Subsequently, the structure shown in FIG. 2C or 2D is heated, i.e.,annealed, using an oxidation process at a temperature at which theimplant oxygen precipitates as oxides, and the precipitated oxidescombine to form a thin, uniform buried oxide layer 18. A lot of thepores in the porous Si-containing region 12 are consumed during thethermal oxidation process and the remaining ones, if any, typicallycollapse into several large voids. In some embodiments in which theinitial Si-containing substrate contains boron as a p-type dopant, theboron diffuses out from the starting substrate during this thermaloxidation step. The resultant structure including buried oxide region 18and Si-containing over-layer 20, i.e., the SOI layer, is shown, forexample, in FIGS. 2E and 2F.

Note that an oxide layer 22 is formed atop the Si-containing over-layer20 during the heating step. This surface oxide layer, i.e., oxide layer22, is typically, but not always, removed from the structure after theheating step using a conventional wet etch process wherein a chemicaletchant such as HF that has a high selectivity for removing oxide ascompared to silicon is employed. FIG. 2G or 2H shows the structure afterthe surface oxide layer 22 has been removed.

The thickness of the buried oxide and the Si-containing over-layer canbe controlled to desired values by adjusting the thermal oxidationconditions. The surface oxide layer 22 formed after the heating step ofthe present invention has a variable thickness which may range fromabout 10 to about 1000 nm, with a thickness of from about 20 to about500 nm being more typical.

Specifically, the heating step of the present invention is a thermaloxidation process that is performed at a temperature from about 650° toabout 1350° C., with a temperature from about 1200° to about 1325° C.being more highly preferred. Moreover, the heating step of the presentinvention is carried out in an oxidizing ambient which includes at leastone oxygen-containing gas such as O₂, NO, N₂O, ozone, air and other likeoxygen-containing gases. The oxygen-containing gas may be admixed witheach other (such as an admixture of O₂ and NO), or the gas may bediluted with an inert gas such as He, Ar, N₂, Xe, Kr, or Ne. When adiluted ambient is employed, the diluted ambient contains from about 0.5to about 100% of oxygen-containing gas, the remainder, up to 100%, beinginert gas.

The heating step may be carried out for a variable period of time thattypically ranges from about 10 to about 1800 minutes (at 1200° to about1325° C.), with a time period from about 60 to about 600 minutes beingmore highly preferred. The heating step may be carried out at a singletargeted temperature, or various ramp and soak cycles using various ramprates and soak times can be employed.

In another embodiment of the present invention wherein excess dopantions are implanted, a post oxidation thermal anneal in a hydrogenambient can be used to reduce the level of dopants within theSi-containing over-layer. When such a post oxidation process isperformed, the post oxidation thermal anneal in a hydrogen ambient isperformed at a temperature from about 800° to about 1200°C., with atemperature from about 1000° to about 1150° C. being more highlypreferred. Examples of hydrogen ambients include H₂, NH₄, and mixturesthereof, including mixtures with, or without, an inert gas. Theconcentration of dopant ions with the Si-containing over-layer may bereduced by more than two orders of magnitude using the aforementionedpost oxidation thermal anneal.

In yet another embodiment of the present invention, the single crystalSi-containing layer is not formed atop the Si-containing substratecontaining the porous Si region. In this embodiment, the oxygen ions aredirectly implanted into the porous Si-containing substrate. Thissubstrate may, or may not have undergone a H₂ bake treatment. Theresulting buried oxide would be still uniform, but somewhat thicker dueto faster oxygen diffusion from the ambient to the buried oxide duringthermal oxidation.

In accordance with the present invention, the Si-containing over-layer20 has a thickness of about 1000 nm or less, with a thickness of fromabout 10 to about 800 nm being more highly preferred. Note that theSi-containing over-layer 20 formed in the present invention is a thinlayer that is substantially defect free. The buried oxide layer 18formed during the heating step has a thickness of about 5 nm to about100 nm, with a thickness of from about 10 to about 80 nm being morehighly preferred. The buried oxide layer 18 has a smooth and continuousinterface with the Si-containing over-layer 20.

As stated above, the surface oxide layer 22 may be stripped at thispoint of the present invention so as to provide the Si-on-insulatorsubstrate material shown, for example, in FIG. 2G or 2H.

FIG. 3A is a cross-sectional SEM micrograph showing a buried oxide layerof approximately 12 nm thickness formed by the inventive methoddescribed here. The substrate process history was as follows:

-   -   Starting substrate: Boron-doped wafers with p-doping of 1E19        cm⁻³    -   Porous-Si Formation: current, 0.5-1.0 mA, time: approximately 2        min    -   Epi-Si growth: 4000-5000 Å with a H₂ bake at 1150° C.    -   Oxygen implant @350° C.: 5E16 cm⁻²    -   Oxygen implant @nominal room temperature: 2E15 cm⁻²    -   High temperature anneal: 10 hrs @1325° C. with approximately 25%        oxygen mixed with Ar+5 hrs @1325° C. with approximately 35%        oxygen mixed with Ar

-   Region A is the surface oxide grown during the anneal

-   Region B is the SOI layer

-   Region C is the thin buried oxide

-   Region D is the substrate

FIG. 3B is a cross-sectional SEM micrograph showing a broken buriedoxide layer of approximately 76 nm islands. This region of the samesubstrate as above did not receive any porous-Si treatment but didreceive the same Si epi growth, oxygen implants and anneals as shown inFIG. 3A. This figure clearly demonstrates that the existence of aporous-Si is the center piece to create a thin and continuous buriedoxide. The substrate history is described below.

-   -   Starting substrate: Boron-doped wafers with p-doping of 1E19        cm⁻³    -   Epi-Si growth: 4000-5000 Å with a H₂ bake at 1150° C.    -   Oxygen implant @350° C.: 5E16 cm⁻²    -   Oxygen implant @nominal room temperature: 2E15 cm⁻²    -   High temperature anneal: 10 hrs @1325° C. with approximately 25%        oxygen mixed    -   with Ar+5 hrs @1325° C. with approximately 35% oxygen mixed with        Ar

-   Region A is the surface oxide grown during the anneal

-   Region B is the SOI layer

-   Region C is that with broken buried oxide

-   Region D is the substrate

FIG. 3C is a cross-sectional SEM micrograph showing how the buried oxidelayer thickness can be controlled by the base oxygen implant dose.Buried oxide of approximately 36 nm was created by the inventive methoddescribed here. The substrate process history was as follows:

-   -   Starting substrate: Boron-doped wafers with p-doping of 1E19        cm⁻³    -   Porous-Si Formation: current, 0.5-1.0 mA, time: approximately 2        min    -   Epi-Si growth: 4000-5000 Å with a H₂ bake at 1150° C.    -   Oxygen implant @350° C.: 1E17 cm²    -   Oxygen implant @nominal room temperature: 2E15 cm⁻²    -   High temperature anneal: 10 hrs @1325° C. with approximately 25%        oxygen mixed with Ar+5 hrs @1325° C. with approximately 35%        oxygen mixed with Ar

-   Region A is the surface oxide grown during the anneal

-   Region B is the SOI layer

-   Region C is the thin buried oxide

-   Region D is the substrate

While the present invention has been particularly shown and describedwith respect to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formsand details may be made without departing from the scope and spirit ofthe present invention. It is therefore intended that the presentinvention not be limited to the exact forms and details described andillustrated, but fall within the scope of the appended claims.

1. A method of fabricating a substrate comprising: forming a porousSi-containing region via electrolytic anodization of a Si-containingregion having n-type or p-type dopant in an upper portion of aSi-containing substrate; forming a single crystal Si-containing layerdirectly on top of said porous Si-containing region by epitaxialdeposition; forming an oxygen implant region by implanting oxygen ionsinto the porous Si-containing region, wherein a peak oxygen content isprovided by the implanting of the oxygen ions, and is located withinsaid porous Si-containing region or at an interface between said singlecrystal Si-containing layer and said porous Si-containing region; andannealing using a thermal oxidation process at a temperature at whichsaid implanted oxygen ions precipitate as oxides, wherein saidprecipitated oxides combine to form a uniform buried oxide layerextending across an entirety of a semiconductor-on-insulator (SOI)substrate, wherein after the annealing portions of the porousSi-containing region located beneath said uniform buried oxide layer nowcontain voids created by a collapse of previously existing pores in theporous Si-containing region, wherein a variation of thickness of saiduniform buried oxide layer across said entirety of said Si-containingsubstrate is less than 30% of a total thickness of said uniform buriedoxide layer, and wherein a Si-containing over-layer is formed from aremaining portion of said single crystal Si-containing layer.
 2. Themethod of claim 1, further comprising annealing saidsilicon-on-insulator (SOI) substrate in a hydrogen containing ambientafter said thermal oxidation process, wherein a level of dopant atoms insaid Si-containing over-layer is reduced during said annealing in saidhydrogen containing ambient.
 3. The method of claim 1, wherein an oxygendose of about 1E17 atoms/cm² or less is employed during said implantingof said oxygen atoms, and wherein said uniform buried oxide layer has athickness of about 100 nm or less.
 4. A method of fabricating asubstrate comprising: forming a porous Si-containing region viaelectrolytic anodization of a Si-containing region having n-type orp-type dopant in an upper portion of a Si-containing substrate; forminga single crystal Si-containing layer directly on top of said porousSi-containing region by epitaxial deposition; forming a plurality ofpatterned oxygen implant regions by implanting oxygen ions into leastthe porous Si-containing region, wherein a peak oxygen content isprovided by the implanting of the oxygen ions, and is located withinsaid porous Si-containing region or at an interface between said singlecrystal Si-containing layer and said porous Si-containing region; andannealing using a thermal oxidation process at a temperature at whichsaid implanted oxygen ions precipitate as oxides, wherein saidprecipitated oxides combine to form a plurality of buried oxide islands,in which a variation in thickness of the buried oxide islands across anentire width of the buried oxide islands is less than 30% of a totalthickness of the buried oxide islands, wherein after the annealing,portions of the porous Si-containing region located beneath saidplurality of the buried oxide islands now contain voids created by acollapse of previously existing pores in the porous Si-containingregion, wherein a Si-containing over-layer is formed from a remainingportion of said single crystal Si-containing layer, and wherein saidporous Si-containing region abuts said single crystal Si-containinglayer around said plurality of the buried oxide islands.
 5. The methodof claim 4, further comprising annealing said silicon-on-insulatorstructure in a hydrogen containing ambient after said thermal oxidationprocess, wherein a level of dopant atoms in said Si-containingover-layer is reduced during said annealing in said hydrogen containingambient.
 6. The method of claim 4, wherein an oxygen dose of about 1E17atoms/cm² or less is employed during said implanting of said oxygenatoms, and wherein said uniform buried oxide layer has a Thickness ofabout 100nm or less.
 7. A method of fabricating a substrate comprising:forming a porous Si-containing region via electrolytic anodization of aSi-containing region having n-type or p-type dopant in an upper portionof a Si-containing substrate; forming a single crystal Si-containinglayer directly on top of said porous Si-containing region by epitaxialdeposition; forming at least one oxygen implant region by implantingoxygen ions into at least the porous Si-containing region, wherein apeak oxygen content is provided by the implanting of the oxygen ions,and is within said porous Si-containing region or at an interfacebetween said single crystal Si-containing layer and said porousSi-containing region; and annealing using a thermal oxidation process ata temperature at which said implanted oxygen ions precipitate as oxides,wherein said precipitated oxides combine to form a uniform buried oxideregion during said annealing, wherein some pores in said porousSi-containing region collapse into voids beneath the uniform buriedoxide regions during said annealing, wherein after the annealing,portions of the porous Si-containing region located beneath said uniformburied oxide layer now contain voids created by a collapse of previouslyexisting pores in the porous Si-containing region, and wherein aSi-containing over-layer is formed from a remaining portion of saidsingle crystal Si-containing layer to provide asemiconductor-on-insulator (SOI) substrate.
 8. The method of claim 7,further comprising annealing said semiconductor-on-insulator (SOI)substrate in a hydrogen containing ambient after said thermal oxidationprocess, wherein a level of dopant atoms in said Si-containingover-layer is reduced during said annealing in said hydrogen containingambient.
 9. The method of claim 7, wherein an oxygen dose of about 1E17atoms/cm² or less is employed during said implanting of said oxygenatoms, and wherein said uniform buried oxide layer has a thickness ofabout 100 nm or less.